Thulium (Tm)-doped ultrafast fiber lasers with emission wavelengths around 2 μm are desirable sources for scientific, industrial, medical, and environmental applications and flexible testbeds for investigating nonlinear pulse dynamics. Although exceptional research attention has been drawn by Tm-doped ultrafast fiber lasers in recent years, their designs and dynamics are significantly less explored compared to other fiber laser systems. Despite the broad emission spectrum of Tm-doped fibers, power scaling of Tm-doped ultrafast fiber lasers has been limited at shorter wavelengths of their emission spectrum (<1920 nm) due to challenges including signal re-absorption. However, compact, high-energy ultrafast sources at these less-exploited wavelengths can enable various applications including nonlinear microscopy. Further, due to the challenges of implementing real-time characterization around 2 μm, transient nonlinear pulse dynamics have rarely been reported from Tm-doped ultrafast fiber lasers. Resolving these dynamics can not only provide insights into new laser designs but also guide the generation of novel pulse profiles which can benefit a wide range of applications depending on their parameters. This dissertation focuses on developing various novel Tm-doped ultrafast fiber laser systems with unprecedented performance: High-energy operation is demonstrated at less-exploited wavelengths and unique waveforms are generated with their nonlinear dynamics investigated in real-time. First, a high-energy (394-nJ) Tm-doped chirped-pulse-amplification fiber laser system is designed and optimized for operation at the wavelength of 1900 nm and supports the generation of 950-nm ultrashort (390-fs) pulses via frequency-doubling. The system represents the highest pulse-energy (138 nJ) in the femtosecond regime for any fiber-based systems around this wavelength to date, which can be highly attractive for two-photon microscopy with spatiotemporal-multiplexing. To gain deeper insights into the operation of ultrafast Tm-doped fiber lasers, various new nonlinear dynamics are investigated by a home-built real-time characterization setup based on dispersive Fourier transform for 2 μm pulses: A new mode-locking regime is demonstrated which can deliver both up-chirped and close-to-chirp-free dissipative pulses with a 10-fold difference in their pulse energies/durations, providing a versatile source that can switch between different pulse profiles. Following that, soliton molecules with unique partial spectral modulation patterns are synthesized based on two dissimilar pulses from the same cavity, which represent an interesting analogy to ‘heteronuclear’ chemical molecules and hold great potential for optical information processing. Further, mode-locking evolution between dissimilar coherent pulses are studied in Tm-doped ultrafast fiber lasers. Finally, combining both high-energy operation and novel waveform-generation, we present a Tm-doped fiber laser source delivering amplified (~ 200 nJ) noise-like pulses without requiring any feedback mechanism. / 2025-09-10T00:00:00Z

<p dir="ltr">Ultrafast optics is an important field of study for chemistry and physics, providing new information on the dynamics of matter and electrons. In this thesis I describe my work on ultrafast laser systems. The two main systems discussed are a high harmonic generated extreme ultraviolet system and a terahertz spectroscopy system. The first half describes the importance and background of both as well as how these systems are set up. The second half describes my work in designing and implanting these systems, and my work in constructing components for the operation of both.</p>

Doctor of Philosophy / Department of Physics / Kristan L. Corwin / We have made significant accomplishments in the development of portable frequency standard inside hollow optical fibers. Such standards will improve portable optical frequency references available to the telecommunications industry. Our approach relies on the development of a stabilized Cr:forsterite laser to generate the frequency comb in the near-IR region. This laser is self referenced and locked to a CW laser which in turn is stabilized to a sub-Doppler feature of a molecular transition. The molecular transition is realized using a hollow core fiber filled with acetylene gas. We finally measured the absolute frequency of these molecular transitions to characterize the references. In this thesis, the major ideas, techniques and experimental results for the development and absolute frequency measurement of the portable frequency references are presented. A prism-based Cr:forsterite frequency comb is stabilized. We have effectively used the prism modulation along with power modulation inside the cavity in order to actively stabilize the frequency comb. We have also studied the carrier-envelope-offset frequency (f0) dynamics of the laser and its effect on laser stabilization. A reduction of f0 linewidth from [similar to]2 MHz to [similar to]20 kHz has also been observed. Both our in-loop and out-of-loop measurements of the comb stability showed that the comb is stable within a part in 10^11 at 1-s gate time and is currently limited by our reference signal. In order to develop this portable frequency standard, saturated absorption spectroscopy is performed on the acetylene v1+v3 band near 1532 nm inside different kinds of hollow optical fibers. The observed linewidths are a factor 2 narrower in the 20 um fiber as compared to 10 um fiber, and vary from 20-40 MHz depending on pressure and power. The 70 um kagome fiber shows a further reduction in linewidth to less than 10 MHz. In order to seal the gas inside the hollow optical fiber, we have also developed a technique of splicing the hollow fiber to solid fiber in a standard commercial arc splicer, rather than the more expensive filament splicer, and achieved comparable splice loss. We locked a CW laser to the saturated absorption feature using a Frequency Modulation technique and then compared to an optical frequency comb. The stabilized frequency comb, providing a dense grid of reference frequencies in near-infrared region is used to characterize and measure the absolute frequency reference based on these hollow optical fibers.

Die Erzeugung immer kürzerer und energiereicherer Laserimpulse ist eine der wichtigsten Aufgaben der Laserphysik, um physikalische Phänomene in bisher unerreichten elektrischen Feldstärkebereichen zugängig zu machen und Beobachtungen auf kleinster Zeitskala zu ermöglichen. Mit Hilfe der Nachkompression verstärkter, in edelgasgefüllten Hohlfasern selbstphasenmodulierter Ti:Saphir-Laserimpulse werden die momentan kürzesten Impulse des sichtbaren Spektralbereiches erzeugt, die nur noch wenige Schwingungszyklen des elektrischen Feldes umfassen. Ebenso notwendig wie ein solcher Schritt der Impulskompression ist der verlässliche Nachweis seines Ergebnisses. Allerdings wächst auch die physikalische und technische Herausforderung einer präzisen und vollständigen Messung des ultrakurzen Laserimpulses mit zunehmender Komplexität und Breite des Impulsspektrums. Die vorliegende Arbeit stellt sowohl auf dem Gebiet der Kompression von Sub-10-fs Impulsen als auch auf dem der vollständigen Charakterisierung solcher Impulse optimierte aber auch neue Verfahrenstechniken vor. In Experimenten an einem zweistufigen Hohlfaserkompressor wird die Erzeugung der momentan kürzesten, nicht adaptiv komprimierten Impulse mit einer Dauer von lediglich 3,8 fs demonstriert. Eine elegante Alternative zu bisherigen Kompressionsmethoden zeigt der Nachweis effektiver Selbstkompression von mJ-Impulsen auf unter 8 fs in einem selbstführenden Edelgasfilament auf. Zur Kontrolle erfolgreicher Impulskompression und für eine phasenempfindliche Untersuchung des Prozesses der Dispersionskompensation über spektrale Bandbreiten von bis zu einer Oktave mussten etablierte Impulsmesstechniken wie das SPIDER- (Spectral Phase Interferometry for Direct Electric-field Reconstruction) und das FROG- (Frequency-Resolved Optical Gating) Verfahren weiterentwickelt werden. So wird mit der Realisierung und vollständigen Analyse interferometrischer FROG-Messungen ein neues phasenempfindliches Impulsmessverfahren vorgestellt. / One challenge of today’s laser physics is the stable compression of more and more intense laser pulses to the shortest possible pulse duration to enable new high-field laser experiments and to investigate fast atomic or molecular dynamics. At present, the shortest laser pulses of the visible spectral region envelop only a few cycles of the electric field. The state of the art method to generate such short pulses behind a Ti:sapphire amplifier laser system is by means of successive steps of spectral broadening inside a gas-filled hollow fibre and dispersion compensation. However, a reliable pulse characterization is as important as the pulse compression. The more spectral bandwidth the pulse covers the more technically challenging is the measurement of the pulse’s electric field structure. In this work, new concepts of compression and characterization of pulses down to durations below 10 fs are demonstrated as well as further optimization of established techniques. Due to modern, chirped-mirror based dispersion compensation pulses as short as 3.8 fs were generated with a two-stage hollow fibre compressor. At present, these are the shortest pulses of the visible spectral region, compressed without adaptive means for dispersion compensation. For the first time the effect of self-compression of mJ-pulses to below 8 fs in a self-guiding noble gas filament is demonstrated experimentally and determined by numerical simulations. Advanced pulse characterization schemes were needed for a phase-sensitive investigation of dispersion compensation and pulse compression of white light pulses. An optimized design of the SPIDER (Spectral Phase Interferometry for Direct Electric-field Reconstruction) technique is demonstrated that facilitates the measurement of the pulse’s spectral phase in case of broadband structured spectra. With the implementation of an interferometric FROG (Frequency-Resolved Optical Gating) a new phase-sensitive pulse characterization method is introduced.

I present several new methods for the characterisation of ultrashort pulses using interferometry. A generalisation of the concatenation algorithm for spectral shearing interferometry enables interferograms taken at multiple shears to be combined. This improves the precision of the reconstructed phase in the presence of detector noise, and enables the relative phase between disjoint spectral components to be obtained without decreasing the spectral resolution. The algorithm is applied to experimental data from two different implementations of spectral shearing interferometry for ultrashort optical pulses. In one, the shears are acquired sequentially, and in the other they are acquired simultaneously. I develop a form of spatio-temporal ultrashort pulse characterisation which performs both spatial and spectral shearing interferometry simultaneously. It requires a similar geometrical setup to common implementations of spectral phase interferometry for direct electric-field reconstruction, but provides complete amplitude and phase characterisation in time and one spatial dimension. I develop the theory of lateral shearing interferometry for spectrally resolved wavefront sensing of extended ultraviolet and soft x-ray pulses generated using high-harmonic generation. A comprehensive set of wavefront measurements of harmonics 13-25 in Krypton show good agreement with theory, validating the technique. I propose and numerically demonstrate quantum-path interferometry mediated by a weak control field for high harmonic generation. This is a general technique for measuring the amplitude and relative phases of each contributing quantum path. The control field perturbatively modulates the phase of each path. The differing sensitivity of each path to the parameters of the control field allows their contributions to be distinguished from one another.

Pour répondre aux questions que pose la faisabilité d’un dispositif de détection d’explosifsà l’aide de la technologie de spectroscopie térahertz, cette thèse a exploré troisaxes. Le premier a consisté à établir une base de données des signatures spectrales (indiceet absorption) d’une large gamme de matériaux d’intérêt pour ce type d’applications,à partir des données expérimentales que nous avons mesurées par spectroscopie dans ledomaine temporel. Nous avons identifié les matériaux montrant une signature spectralesignificative et nous avons aussi étudié l’effet des matériaux de dissimulation.Dans la seconde partie de ce travail, nous avons conçu et construit un banc de spectroscopieultra-large bande destiné à une meilleure identification spectrale des substances. Nousavons identifié les limites techniques de ce type de banc et donné les pistes pour atteindreles performances désirées.Ensuite, nous avons développé et validé un banc de spectroscopie en réflexion, de typegoniométrique, afin de détecter des signaux térahertz diffusés par des matériaux hétérogènes.Finalement, nous avons étudié les plasmons de surface dans le domaine térahertz, en vuede la détection de très faibles quantités de matière. / This PhD work was performed in view of using terahertz electromagnetic signals forthe detection and identification of dangerous and prohibited substances. In a first stage,a database of the terahertz properties (namely refractive index and absorption) of thesesubstances was created from the measurements we performed using terahertz time-domainspectroscopy. Then a large bandwidth terahertz time-domain set up has been built, togetherwith a goniometric-type set up that allows us recording signals scattered by roughor heterogeneous samples. Finally, we studied the excitation and propagation of surfaceplasmons in the terahertz domain, which may be used for the detection of small amountsof matter.

Ce projet de thèse de doctorat est intitulé «Génération du second harmonique dans des cristaux non-linéaires désordonnés: application pour la caractérisation d'impulsions laser ultra-courtes». Il est consacré à l'étude de la génération de deuxième harmonique dans des cristaux ferroélectriques non linéaires formés par une distribution aléatoire de domaines. Ceci conduit à une distribution aléatoire de la susceptibilité non linéaire quadratique (tels que le nitrate de baryum de strontium –SBN- et les cristaux de nitrate de calcium et de calcium) et son application à la caractérisation unique des impulsions laser ultra-courtes. Le principe de base de l'opération est lié au type unique d'émission associé à ces types de cristaux où le second signal harmonique est émis transversalement à la direction de propagation du faisceau. En utilisant la génération transversale de deuxième harmonique à partir de ces cristaux, nous mesurons la durée de l'impulsion, le paramètre chirp et le profil temporel dans une configuration à un seul pulse laser. Cette méthode a été mise en œuvre à la fois dans l'autocorrélation transversale et les schémas transversaux de corrélation croisée pour la mesure des impulsions avec des durées allant de plusieurs dizaines à plusieurs centaines de femtosecondes. Les principaux avantages obtenus avec les techniques développées par rapport à d'autres méthodes traditionnelles comprennent l'élimination de l'exigence de cristaux minces non linéaires pour la génération harmonique, la possibilité d'obtenir une correspondance automatique de phase sans alignement angulaire ou contrôle de la température sur un spectre très large et un processus d'opération simplifié. / The PhD project, entitled «Second harmonic generation in disordered nonlinear crystals: application to ultra-short laser pulse characterization», is devoted to the study of second harmonic generation in nonlinear ferroelectric crystals formed by a random distribution of domains with inverted quadratic nonlinear susceptibility (such as the Strontium Barium Niobate and Calcium Barium Niobate crystals) and its application to the single-shot characterization of ultrashort laser pulses. The basic principle of operation is related to the unique type of emission associated to those kinds of crystals where the second harmonic signal is emitted transversally to the beam propagation direction. Using the transverse second harmonic generation from these crystals we measure the pulse duration, the chirp parameter and the temporal profile in a single-shot configuration. This method has been implemented both in transverse auto-correlation and transverse cross-correlation schemes for the measurement of pulses with durations in the range from several tens up to several hundreds of femtoseconds. The main advantages gained with the developed techniques against other traditional methods include the removal of the requirement of thin nonlinear crystals for harmonic generation, the possibility to get automatic phase matching without angular alignment or temperature control over a very wide spectrum and a simplified operation process. Different types of pulses have been measured in different conditions and the limits of validity of the technique have been explored.

Diese Arbeit untersucht den Zusammenhang von elektronischer und nukleare Anregung beim Raman-Effekt mit der Methode der zeitaufgelösten harten UV-Spektroskopie. Wir verwenden kohärente stimulierte Raman-Streuung, ein Spezialfall der Standard-Raman-Streuung. Bei dieser regt ein hinreichend kurzer kohärenter Lichtimpuls Schwingungen der Kerne an, bei denen die Kerne messbar ausgelenkt werden, wohingegen die Auslenkungen bei normaler Raman-Streuung, wegen deren inkohärenten spontanen Natur, nicht messbar sind. Wir konnten Auslenkung kleiner als 10^-4 A in Echtzeit durch ihren Effekt auf das harte UV-Spektrum nachweisen. Diese Ergebnisse konnten mit Lithiumborhydrid als Probe und nicht-resonanter naher Infrarotstrahlung als Anrege-und harter UV-Strahlung als Abfrageimpuls erzielt werden. Zum Nachweis dieses Prozesses verwenden wir harte UV-Absorptionsspektroskopie an der Lithium K-Kante von Lithiumborhydrid bei 60 eV. Das Absorptionsspektrum besteht aus einem starken exzitonischen Anteil zu Beginn der Absorption und einem Plateau bei höheren Energien. Bei Anregung durch einen NIR-Impuls beobachteten wir eine oszillatorische Änderung des Absorptionsspektrums mit einer Frequenz von 10 THz, was wir der Modulation der interatomaren Abständen durch kohärente Phononen, und die damit einhergehende Modulation der chemischen Umgebung des absorbierenden Atoms, zuschreiben. Harte UV-Spektroskopie, insbesondere bei niedrigen Energien und nahe der Kante, ist hoch sensitiv auf die chemische Umgebung des jeweiligen absorbierenden Atoms. Unsere Resultate erlauben einen faszinierenden, neuen Einblick in die mikroskopische Natur des Raman-Effekts. Sie verbinden einen direkten Nachweis des antreibenden Mechanismus, der induzierten Polarisation, mit einer direkten Beobachtung des Resultats, die oszillatorische Auslenkung der Kerne. Dabei konnten mit harter UV-Spektroskopie nukleare Auslenkungen in der Größenordnung von 10-4 A mit Subpicosekundenzeitauflösung aufgelöst werden. / This thesis explores the subtle interplay between electronic and nuclear excitation in the Raman effect with time resolved XUV absorption spectroscopy. Coherent stimulated Raman scattering, the type of Raman interaction we induce, is a variant of the well known Raman scattering, where a sufficiently short pulse excites nuclear vibrations coherently, i.e. with actual displacement of the nuclei. In standard Raman scattering, due to its incoherent, spontaneous nature, there is no displacement of nuclei. We were able to observe nuclear displacements as small as 10^-4 in real time by their effect on the XUV absorption spectrum. Specifically we studied non-resonant NIR pump XUV probe absorption spectroscopy on lithium borohydride (LiBH_4). In the XUV absorption experiments in this thesis we concentrate on the Lithium K-edge absorption spectrum around 60 eV which consists of a strong excitonic peak at the onset of absorption and a plateau at higher energies. Upon excitation with a NIR pulse we observe oscillatory changes in the absorption spectrum with a frequency of 10 THz, which we identify as the effect of coherent phonon excitations of an external A_g phonon mode. The coherent oscillation changes the distance between Li^+ anions and BH_4^- cations, which modifies the electronic environment around the Li anion. XUV absorption spectroscopy, especially XANES, is highly sensitive to such changes of the chemical environment around the absorbing atom. We use two different approaches to derive the absolute displacement, which are observed in the experiment. Our results allow for a fascinating new insight into Raman scattering as they connect a direct observation of the driving mechanism, the induced polarization, with a direct observation of the outcome the oscillatory nuclear displacement. With XUV absorption spectroscopy nuclear displacements in the order of 10^-4 A were resolved with sub picosecond accuracy in the time domain.

This thesis describes the use of counterpropagating pulse trains to quasi-phase-match high-harmonic generation (HHG). Two novel techniques for generating trains of ultrafast pulses are described and demonstrated. The first method makes use of a birefringent crystal array to split a single pulse into a sequence of pulses. The second method makes use of the time-varying polarisation of a chirped pulse passed through a multiple-order wave plate to generate a train of pulses by the addition of a polariser. It is demonstrated that this second technique can be used to make pulse trains with non-uniform pulse separation by using an acousto-optic programmable dispersive filter to manipulate the higher-order dispersion encountered by the chirped pulse. The crystal array method is used to demonstrate quasi-phase-matching of HHG in a gas-filled capillary, using one and two counterpropagating pulses. Enhancements of up to 60% of the intensity of the 27th harmonic of the 800,nm driving laser light are observed. Information on the spatial and dynamic properties of the HHG process is obtained from measurements of the coherence length in the capillary. Simulations of HHG in a capillary waveguide have been performed. These agree well with the results of the quasi-phase-matching experiments. The effect of mode-beating on the generation process in a capillary and its use as a quasi-phase-matching mechanism are investigated.

Die Methode „interferometric frequency-resolved optical gating“ (iFROG) zur Charakterisierung ultrakurzer Laserimpulse wurde erweitert. Als optische Nichtlinearität werden sowohl die Erzeugung der 2. als auch der 3. Harmonischen (THG) separat verwendet. Eine iFROG-Messung stellt ein inverses Problem dar, bei dem die Amplitude und Phase des elektrischen Feldes des Laserimpulses nur durch einen iterativen Algorithmus rekonstruiert werden kann. In dieser Arbeit wird ein mathematischer Formalismus entwickelt und mit einem evolutionären Optimierungsalgorithmus kombiniert, um einen neuartigen Impuls-Rekonstruktions-Algorithmus für iFROG zu erschaffen. Während iFROG ursprünglich ausschließlich zur Charakterisierung von Laserimpulsen konzipiert wurde, kann die Technik gleichermaßen für spektroskopische Zwecke eingesetzt werden. Wird das nichtlineare Medium in iFROG durch ein Untersuchungsobjekt ersetzt und ein bekannter Laserimpuls erneut charakterisiert, so kann die Antwortfunktion des Untersuchungsobjekts mit einer sub-Femtosekunden-Auflösung entschlüsselt werden. Da für die THG-Variante bisher keine Lösung bekannt ist, ermöglicht der vorgestellte Rekonstruktion-Algorithmus die erstmalige Nutzung von iFROG zur Untersuchung ultraschneller nichtlinearer Effekte dritter Ordnung. Die spektroskopische Fähigkeit von iFROG wird durch das Studium von drei unterschiedlichen physikalischen Systemen (Nanostrukturen) geprüft. In ZnO-Nanostäben wird die Leistungsabhängigkeit der durch Multiphotonenabsorption induzierten Lumineszenz gemessen, wobei nachgewiesen werden konnte, dass diese mit einer Lokalisierung des optischen Nahfelds verknüpft ist. Eine Dreiphotonenresonanz in einem dünnen Titandioxid Film und eine Oberflächenplasmonenresonanz in Au-Nanoantennen führen beide zu einer endlichen Lebensdauer der induzierten Materialpolarisation. Die iFROG-Methode wird verwendet, um die ultraschnelle zeitliche Dynamik dieser Systeme auf der Nanometer- und wenige Femtosekunden-Skala zu messen. / The ultrashort laser pulse characterization method “interferometric frequency-resolved optical gating” (iFROG) is extended. Both second- and third harmonic generation (SHG and THG) are separately employed as the optical nonlinearity. An iFROG measurement represents an inverse problem, where the electric field amplitude and phase of the underlying laser pulse can only be reconstructed by an iterative algorithm. In this work, a mathematical formalism for both the SHG and THG variants of iFROG is developed and combined with an evolutionary optimization algorithm to create a novel pulse retrieval algorithm for iFROG. While iFROG was originally conceived solely for pulse characterization, the technique can equally well be applied for spectroscopic purposes. By replacing the nonlinear medium in iFROG with an object of study, say a nanostructure, and characterizing a known pulse again such that the sample affects the harmonic generation process, the response of the object can be deciphered with sub-femtosecond precision. As no previous solution for the THG variant exists, the presented retrieval algorithm allows iFROG to be exploited in the study of ultrafast third-order nonlinear effects for the first time. The spectroscopic capability of iFROG is put to test by studying three differing physical systems, each consisting of nanostructures resting on dielectric substrates. Subjecting these specimen to few-cycle near-infrared pulses, a rich variety of nonlinear optical phenomena is observed. In ZnO nanorods, the power dependence of multiphoton-absorption induced luminescence is measured and found to be connected to a localization of the optical near-field. A three-photon resonance in a thin film of titania and a localized surface plasmon resonance in Au nanoantennas both lead to a finite lifetime of the induced material polarization. The THG-iFROG method is harnessed to measure the ultrafast temporal dynamics of these systems at the nanometer and few-femtosecond scales.